1. Field of the Invention
The present invention relates generally to computer peripheral devices, and more particularly relates to a circuit arrangement for impedance-matching high frequency signal transmission media interconnecting such peripheral devices.
2. Art Background
Computer systems typically comprise numerous functional computational components and subsystems operating together to produce a desired result. A common arrangement of component subparts forming a computer system might include a central processor unit (CPU), memory devices, input and output (I/O) devices, and optional peripheral devices. The various functional subsystems communicate with each other over various forms of communication pathways. Communication pathways may comprise distributed shared pathways, for example buses, or may comprise dedicated private line pathways. In connection with smaller personal computers and standalone workstations, a common communications pathway is the well known and documented standard Small Computer System Interface (SCSI) bus.
Shared subsystems such as hard disk drives, I/O devices, and peripheral devices typically interact with the CPU over one or more system buses. In present day compact computer systems, for example a personal desktop computer, all memory components including hard disk drives typically are mounted internally within the computer case. in such a case, internal buses and communication lines interconnecting the various subsystems are fairly short, on the order of tens of centimeters. Alternatively, especially in high performance or multiple-user "server" platforms, memory or peripheral devices may be remotely located from the accessing CPU. In the latter case, the external interconnecting buses and communications pathways may become physically long, exceeding several meters.
Signal propagation in a communications pathway physically occurs along an electrically conductive element, e.g., a wire, and depends upon the frequency and length of the conductor. As operating speeds of computer systems increase, propagation of signals between the CPU and memory or peripheral devices over the communications pathways must be increasingly well controlled. For example, at relatively slow system clock and bus data rates, i.e., a few megahertz (MHz), signal propagation characteristics remain relatively independent of the conductor length, the signal waveform being adequately predicted and described by DC circuit analysis. At low frequencies, signals are fully absorbed at media discontinuities and at terminal ends of the conductor, and do not affect other functional subsystems coupled to the conductor, i.e., the bus.
However, as signal frequency increases, e.g., greater than 40 MHz, transmission characteristics are no longer independent of the conductor geometry, especially length. In particular, reflections of transmitted high frequency signals at conductor discontinuities and conductor terminations can create superimposed signals, and thereby cause unexpected results. For example, in the case of high frequency digital signals, consider the case where an incident logic "1", or "HIGH", signal is reflected from a conductor discontinuity or termination. Although the absolute voltage of the reflected signal may comprise a reduced voltage relative to the incident logic "1" signal, the reflected signal may nevertheless be sufficient to still comprise a logic "1" signal. In such a case, a device keying upon the signal could be counted or "clocked" twice: once upon the incident signal, and again upon the reflection. Unexpected or fatal computer malfunctioning may likely follow.
To preclude instances of reflected signals causing data transmission errors in high frequency data communications pathways, line terminators were developed to match the impedance of the transmission line and thus provide a nonreflecting, i.e., an "absorbing", termination to the transmission line. Line terminators are coupled to terminal ends of transmission pathways (e.g., buses) so that signals transmitted on pathways that ordinarily would have open terminations, are fully absorbed at the terminal end of the the pathway. By providing terminators at terminal ends of a bus interconnecting one or more subsystems, reflections on the bus are minimized or precluded altogether, thereby ensuring reliable operation of functional subsystems operating on the bus.
Although the function of terminators is easily described, determining when to use line terminators proves difficult in practice. The foregoing is principally true because a system user must physically attach or detach a line terminator to internal and external segments of the bus depending upon certain signal transmission conditions, which conditions are often not easily ascertained by the user. For example, if no disk drive is connected at the terminal end of an external SCSI bus segment, a line terminator should be attached to prevent undesirable reflections from the unterminated bus end. Alternatively, if the SCSI bus segment is short relative to the signal frequency, e.g., an internal SCSI bus segment interconnecting only one internal peripheral device, a terminator is not required to prevent reflections. Generally, however, one terminator is necessary to act as a pull-up device for the SCSI bus regardless of the length of the SCSI bus. If a user wishes to connect a disk drive to the terminal end of a SCSI bus already terminated by a line terminator, the user must first disconnect the terminator and then connect the disk drive.
More recently, switching terminators have been developed which electrically connect or disconnect a terminator circuit element from the signal pathway, without requiring physical intervention by the user. An example of such a switching terminator is the model MCCS142235, manufactured by Motorola, Inc., Schaumberg, Ill. However, a user must still make a determination whether to engage the terminator in the first instance, and then activate or deactivate the switching terminator accordingly. Further, although software interfaces could be written to enable the switching terminator as necessary, the additional layer of code required to run on the system processor is cumbersome and would be implementation-specific.
As will be described in the following detailed description, the present invention overcomes many of the problems associated with the prior art by providing a voltage sensing circuit arrangement for automatically sensing whether line terminator devices are present at terminal ends of a internal and external interconnecting communications pathways, and thereafter enabling or disabling a switching terminator in accordance with the results of the voltage sensing circuit. The sensing circuit arrangement permits a computer system to automatically adapt high speed communications pathways according to user-implemented configurations, thereby ensuring reliable data signal transmission and subsystem operation without requiring intervention by the user.